fix(scan): reject missing column stats fields

[fallintoplace]

Summary

• validate IncludeColumnStats() column names against the resolved snapshot schema during scan build
• return a validation error when requested stats columns are missing instead of silently dropping them
• leave Schema::Select() and regular scan projection semantics unchanged

Tests

• pre-commit run --all-files
• cmake --build build --target scan_test
• ctest --test-dir build -R '^scan_test$' --output-on-failure

[wgtmac]

Ah, I didn't notice this PR before my agent raised #786. By practice we should respect the 1st PR that works on the same issue. However I think this PR chose the wrong direction (changed the semantics of Schema::Select) and applied a more complex fix (relied on heavy call to Schema::Select for validation). Do you mind changing your PR to use the approach in #786? Then I can merge this one and close that one.

[fallintoplace]

Thank you @wgtmac, I will push the commit in a few hours.

[fallintoplace]

Thanks @wgtmac, pushed the narrower version following #786’s approach.

This now validates missing IncludeColumnStats() columns during scan build and leaves Schema::Select() semantics unchanged. I also updated the PR title/description. CI is green now.

[wgtmac]

Thank you, @fallintoplace!